1. Field of the Invention
The present invention relates to systems and methods for controlling delivery of a pressurized flow of breathable gas to a patient and, more particularly, to a ventilation mask such as a full face mask, nasal mask, nasal prongs mask or nasal pillows mask for use in critical care ventilation, respiratory insufficiency or OSA (obstructive sleep apnea) with CPAP (Continuous Positive Airway Pressure) therapy and incorporating a piloted exhalation valve inside the mask.
2. Description of the Related Art
As is known in the medical arts, mechanical ventilators comprise medical devices that either perform or supplement breathing for patients. Early ventilators, such as the “iron lung”, created negative pressure around the patient's chest to cause a flow of ambient air through the patient's nose and/or mouth into their lungs. However, the vast majority of contemporary ventilators instead use positive pressure to deliver gas to the patient's lungs via a patient circuit between the ventilator and the patient. The patient circuit typically consists of one or two large bore tubes (e.g., from 22 mm ID for adults to 8 mm ID for pediatric) that interface to the ventilator on one end, and a patient mask on the other end. Most often, the patient mask is not provided as part of the ventilator system, and a wide variety of patient masks can be used with any ventilator. The interfaces between the ventilator, patient circuit and patient masks are standardized as generic conical connectors, the size and shape of which are specified by regulatory bodies (e.g., ISO 5356-1 or similar standards).
Current ventilators are designed to support either “vented” or “leak” circuits, or “non-vented” or “non-leak” circuits. In vented circuits, the mask or patient interface is provided with an intentional leak, usually in the form of a plurality of vent openings. Ventilators using this configuration are most typically used for less acute clinical requirements, such as the treatment of obstructive sleep apnea or respiratory insufficiency. In non-vented circuits, the patient interface is usually not provided with vent openings. Non-vented circuits can have single limb or dual limb patient circuits, and an exhalation valve. Ventilators using non-vented patient circuits are most typically used for critical care applications.
Vented patient circuits are used only to carry gas flow from the ventilator to the patient and patient mask, and require a patient mask with vent openings. When utilizing vented circuits, the patient inspires fresh gas from the patient circuit, and expires CO2-enriched gas, which is purged from the system through the vent openings in the mask. This constant purging of flow through vent openings in the mask when using single-limb circuits provides several disadvantages: 1) it requires the ventilator to provide significantly more flow than the patient requires, adding cost/complexity to the ventilator and requiring larger tubing; 2) the constant flow through the vent openings creates and conducts noise, which has proven to be a significant detriment to patients with sleep apnea that are trying to sleep while wearing the mask; 3) the additional flow coming into proximity of the patient's nose and then exiting the system often causes dryness in the patient, which often drives the need for adding humidification to the system; and 4) patient-expired CO2 flows partially out of the vent holes in the mask and partially into the patient circuit tubing, requiring a minimum flow through the tubing at all times in order to flush the CO2 and minimize the re-breathing of exhaled CO2. To address the problem of undesirable flow of patient-expired CO2 back into the patient circuit tubing, currently known CPAP systems typically have a minimum-required pressure of 4 cm H2O whenever the patient is wearing the mask, which often produces significant discomfort, claustrophobia and/or feeling of suffocation to early CPAP users and leads to a high (approximately 50%) non-compliance rate with CPAP therapy.
When utilizing non-vented dual limb circuits, the patient inspires fresh gas from one limb (the “inspiratory limb”) of the patient circuit and expires CO2-enriched gas from the second limb (the “expiratory limb”) of the patient circuit. Both limbs of the dual limb patient circuit are connected together in a “Y” proximal to the patient to allow a single conical connection to the patient mask. When utilizing non-vented single limb circuits, an expiratory valve is placed along the circuit, usually proximal to the patient. During the inhalation phase, the exhalation valve is closed to the ambient and the patient inspires fresh gas from the single limb of the patient circuit. During the exhalation phase, the patient expires CO2-enriched gas from the exhalation valve that is open to ambient. The single limb and exhalation valve are usually connected to each other and to the patient mask with conical connections.
In the patient circuits described above, the ventilator pressurizes the gas to be delivered to the patient inside the ventilator to the intended patient pressure, and then delivers that pressure to the patient through the patient circuit. Very small pressure drops develop through the patient circuit, typically around 1 cm H2O, due to gas flow though the small amount of resistance created by the tubing. Some ventilators compensate for this small pressure drop either by mathematical algorithms, or by sensing the tubing pressure more proximal to the patient.
Ventilators that utilize a dual limb patient circuit typically include an exhalation valve at the end of the expiratory limb proximal to the ventilator, while ventilators that utilize a single limb, non-vented patient circuit typically include an exhalation valve at the end of the single limb proximal to the patient as indicated above. Exhalation valves can have fixed or adjustable PEEP (positive expiratory end pressure), typically in single limb configurations, or can be controlled by the ventilator. The ventilator controls the exhalation valve, closes it during inspiration, and opens it during exhalation. Less sophisticated ventilators have binary control of the exhalation valve, in that they can control it to be either open or closed. More sophisticated ventilators are able to control the exhalation valve in an analog fashion, allowing them to control the pressure within the patient circuit by incrementally opening or closing the valve. Valves that support this incremental control are referred to as active exhalation valves. In existing ventilation systems, active exhalation valves are most typically implemented physically within the ventilator, and the remaining few ventilation systems with active exhalation valves locate the active exhalation valve within the patient circuit proximal to the patient. Active exhalation valves inside ventilators are typically actuated via an electromagnetic coil in the valve, whereas active exhalation valves in the patient circuit are typically pneumatically piloted from the ventilator through a separate pressure source such a secondary blower, or through a proportional valve modulating the pressure delivered by the main pressure source.